The kelp and seaweed extract is compatible with your nutrients, but like anything else you have to be careful how much you add, and account for it in the total EC/TDS value of your feeding solution.
Let’s say you decide a good number for your feedings is 1000 EC or 700 PPM. You want your base nutrients to make up the majority of that number, then you can use different additives (B-52, Big Bud, Bud Candy, Kelp/seaweed extract, ect) to reach your target EC. For example, my plants are currently in week 10 and I’m pushing them pretty hard, feeding at 1200EC. The first 275 of that is Cal/mag (because I’m growing in coco). After the CM, the part A of my base nutrients takes it up to around 625. Then the part B brings it up to about 1000. So that only leaves me room to add 200EC worth of whichever additives I have on my schedule for that day.
My suggestion to get familiar with mixing properly is to pick a good size container for you, (I use 5 gallon water jugs) and then write down exactly how many milliliters/ounces you add of each product, and the EC readings. It’s confusing at first, but before you know it all the readings will become very predictable to you.
In my opinion one of the most important things about mixing nutrients (next to the order, which is critical) is having patience and not rushing. After I add my Cal/mag and shake the jug, I wait at least 5 minutes before taking my reading, and then I wait another 5 minutes and take a reading again before I add the part A of my base nutrients, and I repeat those steps between all the rest of the products I add. If you were to add the part A too soon after the Cal/mag, or add the part B too soon after the A, some of the elements can have unwanted effects on each other and end up making themselves less available to the plants. I’m sure I go a little overboard with caution, but better safe than sorry! It usually takes me about an hour and 15 minutes to mix my solution, but most of that is waiting time between products, and I use those minutes to pet and love my cats who are always jealous of me working with the plants, lol.
If you’re using Bud Candy, then there’s no need for blackstrap molasses. They have the same purpose, but Bud Candy is a better option in my opinion. The molasses isn’t really for the plants directly, it’s to feed the microbial life in your root zone which in turn help your plants stay fed. Bud Candy does the same thing, but it also has a small amount of magnesium in it as well, which helps prevent deficiencies which can be fairly common in soil grows sometimes.
Hope this wasn’t too confusing, I’m known for rambling on a bit too much sometimes!